1. Field of the Invention
The present invention relates to a serial printer such as an ink-jet printer by moving a head forward and backward in main scanning directions with respect to a sheet of paper transported in a sub scanning direction for forming an image on the sheet of paper.
2. Description of the Background Art
Serial printers including an ink-jet printers move a head forward and backward in main scanning directions with respect to a sheet of paper transported in a sub scanning direction for forming an image on the sheet of paper. In an ink-jet printer, for example, ink is emitted from a plurality of nozzles arranged at a head against a sheet of paper. A predetermined space is ensured between a leading edge of the nozzle and the sheet of paper so that an image is properly formed on the sheet of paper by the emitted ink.
In other words, if the head is too close to the sheet of paper, the sheet of paper may be brought into contact with the head, thereby causing head wearing or damage. On the other hand, if there is too much space between the head and the sheet of paper, an image may be marred by a so-called satellite phenomenon. The satellite phenomenon is caused when fine ink droplets emitted from the nozzles of the head following main droplets of the ink improperly adhere to the sheet of paper, at locations apart from the main droplets. More specifically, as shown in FIG. 1A, if the distance between head 101 and sheet of paper 102 is appropriate, fine ink droplets 111 to 113 are emitted against the location where main droplet 110 adheres. In this case, the image on the sheet of paper 102 is not marred. On the other hand, as shown in FIG. 1B, if the distance between head 101 and paper 102 is too long, fine ink droplets 111 to 113 are emitted against the portion other than where the ink main droplets 110 adhere, thereby marring the image on the sheet of paper 102.
Then, in the conventional serial printer, a position of the head has been made adjustable in a direction vertical to the sheet of paper (in a direction of height), so that the position of the head can be adjusted in the height direction in accordance with a thickness of the sheet of paper on which the image is to be formed. Thus, a distance between the sheet of paper and the head is maintained at a constant value regardless of a thickness of the sheet of paper. Normally, such adjustment of the head is performed by an operating member such as a height adjustment lever, which is arranged for example at the side of the apparatus.
In addition, in the serial printer, an image formation scope in the main scanning direction in which the head is to be moved changes with the size of paper on which the image is to be formed. Therefore, to increase the operating efficiency of the printer by reducing the time required for image formation, the head must be moved forward and backward only over the scope in the main scanning direction of the sheet of paper on which the image is to be formed. To this end, a means for detecting a length of the set sheet of paper in the main scanning direction should be provided.
However, provision of a structure which is only used for detecting the length of the sheet of paper in the main scanning direction results in increase in size of the apparatus and cost. Thus, a relatively low-priced small serial printer is not provided with a structure for detecting the length of the sheet of paper in the main scanning direction and, the head is moved forward and backward in the main scanning directions over the entire scope corresponding to a maximum sized sheet of paper on which an image is to be formed.
However, the conventional serial printer is not provided with a simple structure for detecting a set condition for image formation such as a position of the head in the height direction or a paper size. Thus, determination cannot be made as to whether the image forming conditions are properly set. As a result, degradation of the image quality as well as wearing and damage of the apparatus are caused.
For example, an image formation process is performed even when the position of the head adjusted by a user does not correspond to a thickness of the printing sheet of paper, thereby resulting in head wearing or damage as well as degradation of the image quality. Further, if the head is to be moved forward and backward in the main scanning directions over the entire scope of the maximum sized paper during image formation regardless of a width of the sheet of paper on which the image is to be formed, the head is even moved over the scope with no sheet of paper. This disadvantageously increases image production time and decreases operating efficiency. In addition, the apparatus may be subjected to taint damage caused by insertion of a sheet of paper not corresponding to the image data size.